1. Field of the Invention
The present invention relates to a correlation apparatus and method for frequency synchronization in an Orthogonal Frequency Division Multiplexing (OFDM) system. More particularly, the present invention relates to a correlation apparatus and method for enhancing a frequency synchronization performance in a system that uses a Zadoff Chu (ZC) sequence or a Constant Amplitude Zero-AutoCorrelation (CAZAC) sequence as its reference signal.
2. Description of the Related Art
In an Orthogonal Frequency Division Multiplexing (OFDM) transmission scheme, subcarriers are transmitted having orthogonality to one another. In transmission, while the orthogonality between subcarriers should be sustained, a frequency interval between subcarriers is relatively small for the transmission band. Hence, compared to a single carrier transmission, the OFDM transmission is sensitive to the frequency offset caused by an oscillator mismatch between a transmitter and a receiver.
At an OFDM receiver, the frequency offset is estimated in two steps. First, a fine frequency offset smaller than the subcarrier interval is estimated. Secondly, the frequency offset is estimated per subcarrier interval.
In general, to accurately detect the frequency offset of the received signal in an OFDM transmission system, a reference signal such as a preamble is inserted in the head of a frame.
The reference signal is transmitted in a form that is agreed upon by the transmitter and the receiver. The receiver conducts the frequency synchronization using the received signal and the reference signal.
A Zadoff Chu (ZC) sequence, which is a conventional Constant Amplitude Zero-AutoCorrelation (CAZAC) sequence, features good Peak-to-Average Power Ratio (PAPR) and auto-correlation properties.
The ZC sequence is used as the synchronization reference signal in various systems including not only the OFDM transmission system but also Single Carrier-Frequency Domain Equalization (SC-FDE) systems and Ultra Wide Band (UWB) systems.
However, the ZC sequence disadvantageously degrades the correlation characteristic in the conventional coarse frequency offset detection.
That is, using the relative frequency offset, the detection performance of the coarse frequency offset corresponding to an integer part deteriorates.
Accordingly, there is a need for an apparatus and method for improved frequency synchronization in a broadband wireless access communication system